qemu: don't error out when cgroups don't exist

qemu: don't error out when cgroups don't exist

When creating cgroups for vcpu and emulator threads whilst starting a
domain, we explicitly skip creating those cgroups in case priv->cgroup
is NULL (cgroups not supported) because SetAffinity() serves the same
purpose.  If the host supports only some cgroups (the ones we need are
either unmounted or disabled in qemu.conf), we error out with weird
message even though we could continue starting the domain.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1097028

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>